During the reaction to produce HCN a number of nitriles are formed that need to be separated from the final product. These nitriles are ultimately sent to a thermal converter for destruction but also carry valuable HCN with them which represents a product loss. Three issues exist with state of the art methods for producing HCN which are: 1) the loss of HCN with the nitriles that are purged; 2) the additional NOX generation caused by conversion of HCN and nitriles in the thermal converter; and 3) The build-up of nitriles in the HCN refining train which lead to foaming because of their tendency to form two liquid phases with subsequent variation in performance of the refining train leading to production interruption and occasional loss of product purity.
A well-known process for producing HCN is the so called Andrussow process. The Andrussow process is used for gas phase production of HCN from methane, ammonia, and oxygen over a platinum catalyst. Filtered ammonia, natural gas and oxygen are fed into a reactor and heated in the presence of a platinum catalyst to temperatures in the range of 800-1500° C. Typically, the methane is supplied from natural gas, which can be further purified; C2, C3, and higher hydrocarbons (e.g., ethane, ethene, propane, propene, butane, butene, isobutane, etc., collectively termed C2+ hydrocarbons) can be present in natural gas. While air can be used as a source of oxygen, the reaction can also be carried out using undiluted oxygen or oxygen-enriched air (i.e., an oxygen Andrussow process).
In the Andrussow process, the primary reactor output includes hydrogen cyanide, unreacted ammonia, carbon dioxide, and reaction impurities including organonitriles (such as acetonitrile, acrylonitrile, and propionitrile). Typically, the reactor off-gas product stream containing HCN and un-reacted ammonia is quenched in a waste heat boiler to temperatures in the range of about 100-400° C. at the outlet. The cooled reactor off-gas is sent through an ammonia removal process wherein the ammonia is contacted with an acid in water to form the non-volatile ammonium salt of the acid. This is accomplished by contacting the cooled off-gas with an ammonium phosphate solution, phosphoric acid or sulfuric acid to remove the ammonia. From the ammonia absorber the product off-gas is sent through the HCN absorber where cold water is added to entrain the HCN. The HCN-water mixture is then sent to separation and purification equipment to produce an HCN product stream that is as pure as possible and a water stream that is as pure as possible. The substantially pure hydrogen cyanide can be stored in tanks or directly used as a feedstock. The water stream can either be recycled or disposed of.
Among contaminants observed in the Andrussow process are organonitrile compounds, mainly acetonitrile, acrylonitrile, and propionitrile (as mentioned above), which can end up in the separation and purification equipment. When this occurs, the columns must periodically be purged, otherwise column performance deteriorates and the possibility for production of poor quality product or process upsets increases. Normally a continuous purge is needed to avoid process upsets, but this purging results in a loss of up to 2% of HCN production. When acrylonitrile or other organonitriles are among the impurities, their concentration in the aqueous fractionator bottoms can increase to levels wherein phase separation from the aqueous medium can start to occur. Particularly when high concentrations of acrylonitrile are present as phase-separated enrichment, production of poor quality product or process upsets can and does occur more readily.
Thus, a need exists for a method for making HCN with substantially reduced organonitrile concentrations to alleviate the above described problems with current, state of the art HCN production. Moreover, a need exists for a method for making HCN, wherein HCN is not lost through the current process of continuously purging separation and purification equipment.